Broad band-cholesteric liquid crystal film and process for producing the same, circularly polarizing plate, linearly polarizing element, illuminator, and liquid-crystal display

ABSTRACT

A broad band cholesteric liquid crystal film of the present invention comprises a cholesteric liquid crystal film obtained by polymerizing a liquid crystal mixture containing a polymerizable mesogen compound (a), a polymerizable chiral agent (b) and a photopolymerization initiator (c) between two substrates with ultraviolet light, and has a reflection bandwidth of 200 nm or more. The broad band cholesteric liquid crystal film of the present invention has a broad reflection band and is excellent in durability.

TECHNICAL FIELD

The present invention relates to a broad band cholesteric liquid crystalfilm and a manufacturing method therefor. A broad band cholestericliquid crystal film of the present invention is useful as a circularlypolarizing plate (a reflection polarizer). The present invention relatesto a linearly polarizer, a luminaire and a liquid crystal display usingthe circularly polarizing plate.

BACKGROUND ART

Generally, a liquid crystal display has a structure in which a spacebetween glass plates forming transparent electrodes is filled with aliquid crystal and polarizers are arranged before and after the glassplates. A polarizer used in such a liquid crystal display ismanufactured in a procedure in which iodine or a dichloic dye issubjected to be adsorbed to a polyvinyl alcohol film and the film isstretched in a given direction. The polarizer thus manufactured itselfabsorbs light vibrating in one direction and transmits only lightvibrating in the other direction therethrough to thereby producelinearly polarizing light. Therefore, an efficiency of the polarizercould not exceed 50% theoretically, which works as the greatest factorto reduce an efficiency of a liquid crystal display. As the mattersworse about the absorbed light, if a liquid crystal display is operatedwith an increased output of a light source beyond a level, it results ininconveniences that a polarizer is broken down by heat generation due tothermal conversion of absorbed light or that a display quality isdegraded under thermal influence onto liquid crystal layer in a cell.

A cholesteric liquid crystal having a circularly polarized lightseparating function has a selective reflection characteristic reflectingonly circularly polarized light having a direction thereof coincidingwith a helical rotation direction of the liquid crystal and a wavelengthequal to a helical pitch length of the liquid crystal. With thisselective reflection characteristic used, only a specific circularlypolarizing light of natural light in a given wavelength band istransmission-separated and the other light components are reflected andrecycled, thereby enabling a polarizing film with a high efficiency tobe manufactured. In the context, transmitted circularly polarized lightpasses through a λ/4 plate and thereby converted to linearly polarizinglight, and coincidence of a direction of the linearly polarized lightwith a transmission direction of an absorption polarizer used in aliquid crystal display enables a liquid crystal display with a hightransmittance to be realized. That is, in a case where a cholestericliquid crystal film is combined with a λ/4 plate and the combination isused as a linearly polarizer, the linearly polarizer could achieve abrightness twice as that of a conventional absorption polarizer singlyused, which absorbs 50% of incident light, due to no light losstheoretically.

There has been, however, difficulty in covering all the range of visiblelight, since a selective reflection characteristic of a cholestericliquid crystal is restricted to only a specific wavelength band. Aselective reflection wavelength bandwidth Δλ is expressed by followingformula:Δλ=2λ•(n _(e) −n _(o))/(n _(e) +n _(o))where n_(o): ordinary light refractive index of a cholesteric liquidcrystal molecule, ne: extraordinary light refractive index of thecholesteric liquid crystal molecule, and λ: central wavelength inselective reflection.

The selective reflection wavelength bandwidth Δλ depends on a molecularstructure of the cholesteric liquid crystal itself. According to theabove formula, if (n_(e)−n_(o)) is larger, a selective reflectionwavelength bandwidth Δλ can be broader, while (n_(e)−n_(o)) is usually0.3 or less. With this value being larger, other functions as a liquidcrystal (such as alignment characteristic, a liquid crystal temperatureor the like) becomes insufficient, causing its practical use to bedifficult. Therefore, a selective reflection wavelength bandwidth Δλ hasbeen actually about 150 nm at highest. A cholesteric liquid crystalavailable in practical aspect has had a selective reflection wavelengthbandwidth Δλ only in the range of about 30 to 100 nm in many cases.

A selective reflection central wavelength λ is given by the followingformula:λ=(n _(e) +n _(o))P/2where P: helical pitch length required for one helical turn ofcholesteric liquid crystal.

With a given pitch length, a selective reflection central wavelength λdepends on an average refractive index and a pitch length of a liquidcrystal molecule.

Therefore, in order to cover all the range of visible light, there havebeen adopted methods, in one of which plural layers having respectivedifferent selective reflection central wavelengths are laminated, and inanother of which a pitch length is continuously changed in the thicknessdirection to thereby form a positional distribution of selectivereflection central wavelengths.

For example, there can be exemplified a method in which a pitch lengthis continuously changed in the thickness direction (for example, see apublication of JP-A No. 6-281814, a specification of JP No. 3272668 anda publication of JP-A No. 11-248943). This method is such that when acholesteric liquid crystal composition is ultraviolet exposure-cured,exposure intensities on sides of exposure and light emission aredifferentiated therebetween to alter a polymerization speedtherebetween, which provides a change in compositional ratio of a liquidcrystal composition having a different reaction speed in the thicknessdirection.

The bottom line of this method lies in that exposure intensities onsides of exposure and light emission are greatly different therebetween.Therefore, in many of the examples of the prior art described above,there has been adopted a method in which an ultraviolet absorbent ismixed into a liquid crystal composition so as to cause absorptionthereof in the thickness direction to thereby amplify a difference inexposure dosage according to an optical path length.

A cholesteric liquid crystal film obtained by supplementary examiningthe prior art described above, however, showed a phenomenon that duringa durability test (a heating test or a humidification test), anultraviolet absorbent is precipitated on the surface of the cholestericliquid crystal film or on the laminating interface to another layer. Itis estimated that the ultraviolet absorbent move in the film and coheredin a long-term durability test because of the low molecular thereof. Foruse in general industrial materials, such the precipitation on thesurface is not recognized as a wrong appearance, or even uponprecipitation on the interface is not so problematic as to causeinterfacial release. However, the cholesteric liquid crystal film usedin a liquid crystal display is positioned in a light path of strongtransmitted light, so that when such precipitations are generated, theprecipitated particles are not only directly visualized, but also causeoptical problems such as a reduction in the efficiency of utilization oflight due to cancellation of polarized light by the precipitations, achange in light scatter distribution of a light source due to hazegenerated by the precipitations.

Insofar as the cholesteric liquid crystal film is used in an ordinarytemperature atmosphere, generation of these precipitations is originallyhardly brought about. However, when the cholesteric liquid crystal filmis integrated and used in a liquid crystal display, a radiation heatfrom a light source in backlight is so strong that the precipitations ofthe ultraviolet absorbent is inevitable upon exposure to the heat for along time. Such the precipitations when precipitated uniformly on thesurface are hardly visible and hardly recognizable as defect, but theradiation heat from the light source varies highly on the surface of theliquid crystal display, and the precipitations are increased on only aregion where the radiation heat was intensively applied, and are thusoften recognized as irregularity on the surface. In addition, therequired display brightness of liquid crystal displays in recent yearsis higher than 200 candelas, and the liquid crystal display on the sideof the light source is exposed to light having an intensity of about10,000 candelas. Depending on the temperature in the use environment,heat at about 40 to 60° C. is applied continuously to the liquid crystaldisplay on the side of a light source. Accordingly, the precipitation ofthe ultraviolet absorbent was recognized not only in a heatingreliability test but also in a continuous lighting test of the liquidcrystal film mounted in a liquid crystal display. For example, if aUV-ray cured polymer obtained from a cholesteric liquid crystalcomposition blended with an ultraviolet absorbent is placed in anenvironment of 80° C.×500 hours or 60° C., 90% RH×500 hours, thencloudiness, an increase in haze, and precipitation of powder on thesurface were significantly observed.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a broad bandcholesteric liquid crystal film having a wide reflection band and amanufacturing method therefor. It is also an object of the presentinvention to provide a broad band cholesteric liquid crystal film havinga wide reflection band, and excellent durability and a manufacturingmethod therefor.

It is another object of the present invention to provide a circularlypolarizing plate using the broad band cholesteric liquid crystal film,and furthermore, to provide a linearly polarizer, a luminaire and aliquid crystal display using the circularly polarizing plate.

The present inventors have conducted serious studies in order to solvethe problems with resultant findings that the objects can be achievedwith the following broad band cholesteric liquid crystal film and amanufacturing method therefor, leading to completion of the presentinvention.

That is, the present invention is related to a broad band cholestericliquid crystal film comprising: a cholesteric liquid crystal filmobtained by polymerizing a liquid crystal mixture containing apolymerizable mesogen compound (a), a polymerizable chiral agent (b) anda photopolymerization initiator (c) between two substrates withultraviolet light, and has a reflection bandwidth of 200 nm or more.

A broad band cholesteric liquid film of the present invention describedabove is obtained by polymerizing a polymerizable liquid crystal mixtureand has a broad bandwidth of selective reflection wavelength not yetfounded which is as broad as 200 nm or more. The reflection bandwidth ispreferably 300 nm or more and more preferably 400 nm or more. Areflection bandwidth of 200 nm or more preferably lies in a visiblelight region, especially a wavelength region from 400 to 800 nm.

Note that a reflection bandwidth is a reflection band having reflectanceof a half of the maximum reflectance in a reflectance spectrum of abroad band cholesteric liquid crystal film measured with aspectrophotometer (Instant Multiphotometry System Model No. MCPD-2000,manufactured by Otsuka Electronics Co., Ltd.).

In the above broad band cholesteric liquid crystal film, a pitch lengthin the cholesteric liquid crystal film preferably changes so as tonarrow continuously from a side irradiated with ultraviolet light.

Also in the above broad band cholesteric liquid crystal film,preferably, the polymerizable mesogen compound (a) has one polymerizablefunctional group and the polymerizable chiral agent (b) has two or morepolymerizable functional groups.

As disclosed by Broer et al. in Nature, 378, p. 467 (1995), a broad bandcholesteric liquid crystal film diffusing a mesogen compound havingchirality and changing the pitch continuously is obtained. In thepresent invention, on the other hand, a broad band cholesteric liquidcrystal changing the pitch continuously is obtained by diffusing amesogen compound having one polymerizable functional group, and thus theorder of change of the chiral pitch is reversed. That is, a broad bandcholesteric liquid crystal film having such pitch change as to narrowthe pitch length continuously from the side of ultraviolet lightirradiation can be obtained in the present invention. The pitch lengthis preferably changed such that the difference in pitch length betweenthe side of ultraviolet light irradiation and the opposite side is madeat least 100 nm. The pitch length was read from a sectional TEM image ofthe broad band cholesteric liquid crystal film.

The liquid crystal mixture forming the broad band cholesteric liquidcrystal film may not contain an ultraviolet absorbent.

According to the present invention, a broad band cholesteric liquidcrystal film having a broad reflection bandwidth can be obtained withoutusing an ultraviolet absorbent. Accordingly, the broad band cholestericliquid crystal film of this invention is free from an increase in haze,a reduction in transmittance of polarized light, and visualization ofprecipitated particles, caused by use of an ultraviolet absorbent, isexcellent in durability in a heating/humidification environment, and issuperior in reliability.

The molar absorption coefficient of the polymerizable mesogen compound(a) in the broad band cholesteric liquid crystal film is preferably 50to 500 dm³·mol⁻¹·cm⁻¹ at 365 nm. The liquid crystal film having thismolar absorption coefficient has an ability to absorb UV rays. The molarabsorption coefficient is more preferably 100 to 250 dm³·mol⁻¹·cm⁻¹ at365 nm. When the molar absorption coefficient is lower than 50dm³·mol⁻¹·cm⁻¹ at 365 nm, the difference of the polymerization rate issmall, thus it is hard to make the broad band liquid crystal film. Onthe other hand, when the molar absorption coefficient is higher than 500dm³·mol⁻¹·cm⁻¹ at 365 nm, the polymerization does not completelyprogress, so curing may be not finished. The molar absorptioncoefficient is a value determined from absorbance at 365 nm in ameasured spectrophotometric spectrum of each material.

The polymerizable mesogen compound (a) is preferably a compoundrepresented by the following general formula (1):

wherein R₁ represents a hydrogen atom or a methyl group, and n is aninteger of 1 to 5.

The present invention also related to a manufacturing method for theabove broad band cholesteric liquid crystal film comprising steps of:polymerizing a liquid crystal mixture containing a polymerizable mesogencompound (a), a polymerizable chiral agent (b) and a photopolymerizationinitiator (c) between two substrates with ultraviolet light. The broadband cholesteric liquid crystal film of this invention can be producedby regulating the temperature, ultraviolet light intensity andirradiation time in ultraviolet light irradiation.

The present invention also related to a circularly polarizing platecomprising the above broad band cholesteric liquid crystal film.

The present invention also related to a linearly polarizer comprisingthe above circularly polarizing plate and a λ/4 plate laminating on thecircularly polarizing plate. In the linearly polarizer, the circularlypolarizing plate, which is the cholesteric liquid crystal film,preferably laminates on the λ/4 plate so that a pitch length in the filmis narrowed toward the λ/4 plate continuously.

The present invention also related to a linearly polarizer comprising anabsorption polarizer adhering to the above linearly polarizer so that atransmission axis direction of the absorption polarizer and atransmission axis of the above linearly polarizer are arranged inparallel with each other.

The λ/4 plate used in the linearly polarizer preferably satisfies that aNz coefficient defined by formula (nx−nz)/(nx−ny) is −0.5 to −2.5 whenin-plane major refractive indexes are nx and ny respectively and themajor refractive index in the direction of thickness is nz.

The present invention also related to a luminaire comprising the abovecircularly polarizing plate or the above linearly polarizer on a frontsurface side of a surface light source having a reflective layer on theback surface side thereof.

The present invention further related to a liquid crystal displaycomprising a liquid crystal cell in a light emitting side of the aboveluminaire.

The linearly polarizer, the luminaire and the liquid crystal display canbe used adhering the each forming layers wholly or partially to oneanother via an adhesive layer.

The broad band cholesteric liquid crystal film of this invention is usedas a circularly polarizing plate and can give a linearly polarizer bycombination with a λ/4 plate. Further, the liquid crystal display canimprove reliability for example by combination with an absorptionpolarizer.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a conceptual view of a polarizing plate used in Example 1 to 3and Comparative Example 1 to 3, wherein a numerical symbol (1) indicatesan polarizing plate, (2) a λ/4 plate, (3) a cholesteric liquid crystalfilm (circularly polarizing plate), (4) a pressure sensitive adhesivelayer.

FIG. 2 is a reflectance spectrum of a cholesteric liquid crystal filmmanufactured in Example 1.

FIG. 3 is a reflectance spectrum of a cholesteric liquid crystal filmmanufactured in Example 2.

FIG. 4 is a reflectance spectrum of a cholesteric liquid crystal filmmanufactured in Example 3.

FIG. 5 is a reflectance spectrum of a cholesteric liquid crystal filmmanufactured in Comparative Example 1.

FIG. 6 is a reflectance spectrum of a cholesteric liquid crystal filmmanufactured in Comparative Example 2.

FIG. 7 is a reflectance spectrum of a cholesteric liquid crystal filmmanufactured in Comparative Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

A cholesteric liquid crystal film of the present invention is obtainedby ultraviolet polymerizing a liquid crystal mixture containing apolymerizable mesogen compound (a), a polymerizable chiral agent (b) anda photopolymerization initiator (c).

A polymerizable mesogen compound (a) preferably has at least onepolymerizable functional group and in addition, a mesogen groupcontaining a ring unit and others. As polymerizable functional groups,exemplified are an acryloyl group, a methacryloyl group, an epoxy group,a vinyl ether group and others, among which preferable are an acryloylgroup and a methacryloyl group. The broad band cholesteric liquidcrystal film, as described above, the polymerizable mesogen compound (a)preferably has the molar absorption coefficient of 50 to 500dm³·mol⁻¹·cm⁻¹ at 365 nm. As a polymerizable mesogen compound (a) havingthe above molar absorption coefficient, as described above, is acompound represented by the following general formula (1):

wherein R₁ represents a hydrogen atom or a methyl group, and n is aninteger of 1 to 5.

As concrete examples of the above polymerizable mesogen compound (a),exemplified are the following compounds:

As a polymerizable chiral agent (b), exemplified is LC756 manufacturedby BASF Corp.

A mixing amount of a polymerizable chiral agent (b) is preferably in therange of about from to 20 parts by weight and more preferably in therange of from 3 to 7 parts by weight relative to 100 parts by weight ofa total amount of a polymerizable mesogen compound (a) and thepolymerizable chiral agent (b). A helical twist power (HTP) iscontrolled by a ratio of a polymerizable mesogen compound (a) and apolymerizable chiral agent (b). By adjusting the proportion within therange, a reflection band can be selected so that a reflectance spectrumof an obtained cholesteric liquid crystal film can cover all the rangeof visible light.

Any kind of photopolymerization initiators (c) can be employed withoutimposing any specific limitation thereon. Exemplified are IRGACURE 184,IRGACURE 907, IRGACURE 369, IRGACURE 651 and others manufactured byChiba Specialty Chemicals Corp. A mixing amount of a photopolymerizationinitiator is preferably in the range of about from 0.01 to 10 parts byweight and more preferably in the range of from 0.05 to 5 parts byweight relative to 100 parts by weight of a total amount of apolymerizable mesogen compound (a) and a polymerizable chiral agent (b).

In the present invention, a liquid crystal mixture containing apolymerizable mesogen compound (a), a polymerizable chiral agent (b) anda photopolymerization initiator (c) can be used as a solution obtainedby dissolving the mixture into a solvent. Without a specific limitationimposed, preferable as solvents used are methyl ethyl ketone,cyclohexanone, cyclopentanone and others. A concentration of a solutionis usually in the range of about from 3 to 50 weight %.

Manufacture of a cholesteric liquid crystal film of the presentinvention is implemented by polymerizing the liquid crystal mixturebetween two substrates with ultraviolet light.

As substrates, there can be adopted conventionally known members asones. Exemplified are: a rubbing film obtained by subjecting a thin filmmade of polyimide, polyvinyl alcohol or the like formed on a substrateto a rubbing treatment with rayon cloth; an obliquely deposition film;optically oriented film obtained by illuminating a polymer havingphotocrosslinking group such as cynnamate, azobenzene or the like or apolyimide with polarized ultraviolet; and a stretched film and others.Orientation can be implemented by application of a magnetic field, anelectric field and a shearing stress.

Examples of the substrate that are used include: films made of plasticssuch as polyethylene phthalate, triacetyl cellulose, norbornen resin,polyvinyl alcohol, polyimide, polyallylate, polycarbonate, polysulfone,polyethersulfone and others; a glass plate, a quartz sheet and others.

A liquid crystal mixture described above is coated on one substrate andthereafter, the other substrate is laminated on the coat. In case wherethe liquid crystal mixture is a solution, the solution is coated on onesubstrate and the coat is dried, followed by laminating the othersubstrate on the coat. A drying temperature for evaporating a solventhas only to be a temperature of the boiling temperature of the solventor higher. The temperature is only required to be set usually in therange of 80 to 160° C. according to a kind of a solvent.

The coating thickness of the liquid crystal mixture (or in the case of asolution, the coating thickness after removing a solvent by drying) ispreferably about 5 to 20 μm, more preferably about 7 to 12 μm. When thecoating thickness is less than 5 μm, a helical pitch enough to cover areflection bandwidth of 200 nm or more may not be able to formed, whilewhen the coating thickness is more than 20 μm, the orientationregulating force may not sufficiently act, resulting in imperfectorientation.

The polymerization temperature in ultraviolet light irradiation isgenerally preferably 140° C. or less. Specifically, the polymerizationtemperature is preferably about 60 to 140° C., more preferably 80 to120° C. In heating, there is an effect of accelerating the rate ofdiffusion of the monomer component. When the temperature is lower than60° C., the rate of diffusion of the polymerizable mesogen compound (a)is very low, thus requiring a very long time to make a broad band liquidcrystal film.

The ultraviolet light intensity is preferably 0.1 to 20 mW/cm², morepreferably 1 to 10 mW/cm². It is not preferable that the ultravioletlight intensity is higher than 20 mW/cm², because the rate ofpolymerization reaction becomes higher than the rate of diffusion, thusfailing to make a broad band liquid crystal film. The irradiation timeis a time as short as 5 minute or less, preferably 3 minutes or less,extremely preferably 1 minute or less.

A thus obtained cholesteric liquid crystal film may be used either notbeing separated from a substrate or being separated therefrom.

A broad band cholesteric liquid crystal film of the present invention isused as a circularly polarizing plate. A circularly polarizing platewith a λ/4 plate laminated thereon can be used as a linearly polarizer.A cholesteric liquid crystal film, which is a circularly polarizingplate, is preferably laminated on a λ/4 plate so that a pitch length inthe film is narrowed toward the λ/4 plate continuously.

The λ/4 plate is preferable that a Nz coefficient defined by formula(nx−nz)/(nx−ny) satisfies −0.5 to −2.5 when in-plane major refractiveindexes are nx and ny respectively and the major refractive index in thedirection of thickness is nz.

As λ/4 plates, exemplified are: a birefringent film obtained bystretching a film made of a proper plastic such as polycarbnate,norbornen resin, polyvinyl alcohol, polystyrene, polymethylmethacrylate,polypropylene, other polyolefins, polyallylate, polyimide; a alignedfilm made of a liquid crystal material such as a liquid crystal polymer;an aligned layer of a liquid crystal material supported by a film; andothers. A thickness of a λ/4 plate is usually preferably in the range offrom 0.5 to 200 μm and especially preferably in the range of from 1 to100 μm.

A retardation plate functioning as a λ/4 plate in a broad wavelengthrange such as a visible light region can be obtained, for example, by ascheme to superimpose a retardation layer functioning as a λ/4 plate fora monochromatic light of wavelength of 550 nm and a retardation layerexhibiting another retardation characteristic, for example a retardationlayer functioning as a λ/2 plate on each other or the like scheme.Therefore, a retardation plate arranged between a polarizing plate and abrightness enhancement improving film may be made of either oneretardation layer, or two or more retardation layers.

An absorption polarizer is adhered to the linearly polarizer, so that atransmission axis direction of the linearly polarizer are arranged inparallel with each other.

The polarizer is not limited especially but various kinds of polarizermay be used. As a polarizer, for example, a film that is uniaxiallystretched after having dichromatic substances, such as iodine anddichromatic dye, absorbed to hydrophilic high molecular weight polymerfilms, such as polyvinyl alcohol type film, partially formalizedpolyvinyl alcohol type film, and ethylene-vinyl acetate copolymer typepartially saponified film; poly-ene type orientation films, such asdehydrated polyvinyl alcohol and dehydrochlorinated polyvinyl chloride,etc. may be mentioned. In these, a polyvinyl alcohol type filmcontaining dichromatic materials such as iodine is suitably used.Although thickness of polarizer is not especially limited, the thicknessof about 5 to 80 μm is commonly adopted.

A polarizer that is uniaxially stretched after a polyvinyl alcohol typefilm dyed with iodine is obtained by stretching a polyvinyl alcohol filmby 3 to 7 times the original length, after dipped and dyed in aqueoussolution of iodine. If needed the film may also be dipped in aqueoussolutions, such as boric acid and potassium iodide. Furthermore, beforedyeing, the polyvinyl alcohol type film may be dipped in water andrinsed if needed. By rinsing polyvinyl alcohol type film with water,effect of preventing un-uniformity, such as unevenness of dyeing, isexpected by making polyvinyl alcohol type film swelled in addition thatalso soils and blocking inhibitors on the polyvinyl alcohol type filmsurface may be washed off. Stretching may be applied after dyed withiodine or may be applied concurrently, or conversely dyeing with iodinemay be applied after stretching. Stretching is applicable in aqueoussolutions, such as boric acid and potassium iodide, and in water bath.

A polarizing plate on which a transparent protective film prepared onone side or both sides of the polarizer is used. Materials of thetransparent protective, excellent in transparency, mechanical strength,heat stability, water shielding property, isotropy etc., is may bepreferably used. As transparent protective films, for example,transparent polymer films made of polyester type polymers, such aspolyethylene terephthalate and polyethylenenaphthalate; cellulose typepolymers, such as diacetyl cellulose and triacetyl cellulose;polycarbonate type polymer; acrylics type polymer, such as polymethylmethacrylate may be mentioned. Besides, as examples of thetransparent polymer films made of styrene type polymers, such aspolystyrene and acrylonitrile-styrene copolymer; polyolefin typepolymers, such as polyethylene, polypropylene, polyolefin that hascyclo-type or norbornen structure, ethylene-propylene copolymer; vinylchloride type polymer; amide type polymers, such as nylon and aromaticpolyamide may be mentioned. Besides, as examples of the transparentpolymer films made of imide type polymers; sulfone type polymers;polyether sulfone type polymers; polyether-ether ketone type polymers;poly phenylene sulfide type polymers; vinyl alcohol type polymer;vinylidene chloride type polymers; vinyl butyral type polymers; allylatetype polymers; polyoxymethylene type polymers; epoxy type polymers; orblend polymers of the above-mentioned polymers may be mentioned.Especially, preferable when being used is a film made of a transparentpolymer with less of optical birefringence. Preferable from theviewpoint of a protective film for a polarizing plate are triacetylcellulose, polycarbonate, acrylics type polymer, a cyclo-olefine typeresin, polyolefin having a norbornen structure and others.

Moreover, as is described in Japanese Patent Laid-Open Publication No.2001-343529 (WO 01/37007), polymer films, for example, resincompositions including (A) thermoplastic resins having substitutedand/or non-substituted imido group is in side chain, and (B)thermoplastic resins having substituted and/or non-substituted phenyland nitrile group in sidechain may be mentioned. As an illustrativeexample, a film may be mentioned that is made of a resin compositionincluding alternating copolymer comprising iso-butylene and N-methylmaleimide, and acrylonitrile-styrene copolymer. A film comprisingmixture extruded article of resin compositions etc. may be used.

As a transparent protective film preferably used, in viewpoint ofpolarization property and durability, triacetyl cellulose film whosesurface is saponificated with alkali is suitable. In general, athickness of a transparent protective film is about from 10 to 500 μm,preferably from 20 to 300 μm, and especially preferably from 30 to 200μm.

Moreover, it is preferable that the transparent protective film may haveas little coloring as possible. Accordingly, a protective film having aretardation value in a film thickness direction represented byRth=[(nx+ny)/2−nz]×d of from −90 nm to +75 nm (where, nx and nyrepresent principal indices of refraction in a film plane, nz representsrefractive index in a film thickness direction, and d represents a filmthickness) may be preferably used. Thus, coloring (optical coloring) ofpolarizing plate resulting from a protective film may mostly becancelled using a protective film having a retardation value (Rth) offrom −90 nm to +75 nm in a thickness direction. The retardation value(Rth) in a thickness direction is preferably from −80 nm to +60 nm, andespecially preferably from −70 nm to +45 nm.

The transparent protective films on the front and back sides may also betransparent protective films made of either the same polymer material orrespective different polymer materials.

A hard coat layer may be prepared, or antireflection processing,processing aiming at sticking prevention, diffusion or anti glare may beperformed onto the face on which the polarizing film of the abovedescribed transparent protective film has not been adhered.

A hard coat processing is applied for the purpose of protecting thesurface of the polarizing plate from damage, and this hard coat film maybe formed by a method in which, for example, a curable coated film withexcellent hardness, slide property etc. is added on the surface of theprotective film using suitable ultraviolet curable type resins, such asacrylic type and silicone type resins. Antireflection processing isapplied for the purpose of antireflection of outdoor daylight on thesurface of a polarizing plate and it may be prepared by forming anantireflection film according to the conventional method etc. Besides, asticking prevention processing is applied for the purpose of adherenceprevention with adjoining layer.

In addition, an anti glare processing is applied in order to prevent adisadvantage that outdoor daylight reflects on the surface of apolarizing plate to disturb visual recognition of transmitting lightthrough the polarizing plate, and the processing may be applied, forexample, by giving a fine concavo-convex structure to a surface of theprotective film using, for example, a suitable method, such as roughsurfacing treatment method by sandblasting or embossing and a method ofcombining transparent fine particle. As a fine particle combined inorder to form a fine concavo-convex structure on the above-mentionedsurface, transparent fine particles whose average particle size is 0.5to 50 μm, for example, such as inorganic type fine particles that mayhave conductivity comprising silica, alumina, titania, zirconia, tinoxides, indium oxides, cadmium oxides, antimony oxides, etc., andorganic type fine particles comprising cross-linked of non-cross-linkedpolymers may be used. When forming fine concavo-convex structure on thesurface, the amount of fine particle used is usually about 2 to 50weight parts to the transparent resin 100 weight parts that forms thefine concavo-convex structure on the surface, and preferably 5 to 25weight part. An anti glare layer may serve as a diffusion layer (viewingangle magnifying function etc.) for diffusing transmitting light throughthe polarizing plate and magnifying a viewing angle etc.

In addition, the above-mentioned antireflection layer, stickingprevention layer, diffusion layer, anti glare layer, etc. may be builtin the protective film itself, and also they may be prepared as anoptical layer different from the protective layer.

Lamination of the linearly polarizer and further lamination of variouskinds of optical layers can be carried out in a system of laminatingsuch layers successively in the process for producing a liquid crystaldisplay etc., but the previously laminated layers have advantages suchas excellent quality stability, assembling operation etc. andimprovement in the process for producing a liquid crystal display etc.In lamination, a suitable adhesion means such as a pressure sensitiveadhesive layer etc. can be used. In this adhesion, the optical axisthereof can have a suitable arrangement angle depending on intendedretardation characteristics.

The linearly polarizer described above can be provided with a pressuresensitive adhesive layer for adhering itself to another member such as aliquid crystal cell or the like. As pressure sensitive adhesive thatforms adhesive layer is not especially limited, and, for example,acrylic type polymers; silicone type polymers; polyesters,polyurethanes, polyamides, polyethers; fluorine type and rubber typepolymers may be suitably selected as a base polymer. Especially, apressure sensitive adhesive such as acrylics type pressure sensitiveadhesives may be preferably used, which is excellent in opticaltransparency, showing adhesion characteristics with moderatewettability, cohesiveness and adhesive property and has outstandingweather resistance, heat resistance, etc.

In addition to the above description, a pressure sensitive adhesivelayer low in moisture absorption rate and excellent in heat resistanceis preferable from the viewpoints of prevention of a foaming phenomenonand peeling-off phenomenon due to moisture absorption, prevention ofdegradation in optical characteristic and warp of a liquid crystal celldue to the difference of thermal expansion or the like and further, inconsideration of formability of a high quality liquid crystal displayexcellent in durability and the like.

The pressure sensitive adhesive layer may contain additives, forexample, such as natural or synthetic resins, tackifier, glass fibers,glass beads, metal powder, fillers comprising other inorganic powderetc., pigments, colorants and antioxidants. Moreover, it may be apressure sensitive adhesive layer that contains fine particle and showsoptical diffusion nature.

Proper method may be carried out to attach a pressure sensitive adhesivelayer. As an example, about 10 to 40 weight % of the pressure sensitiveadhesive solution in which a base polymer or its composition isdissolved or dispersed, for example, toluene or ethyl acetate or a mixedsolvent of these two solvents is prepared. A method in which thissolution is directly applied on a polarizer using suitable developingmethods, such as flow method and coating method, or a method in which anadhesive layer is once formed on a separator, as mentioned above, and isthen transferred on a an optical film may be mentioned. A pressuresensitive adhesive layer may be prepared with two or more layers whichare made of different compositions or kinds with each layer. Thicknessof an adhesive layer may be suitably determined depending on a purposeof usage or adhesive strength, etc., and generally is 1 to 500 μm,preferably 5 to 200 μm, and more preferably 10 to 100 μm.

A temporary separator is attached to an exposed side of a pressuresensitive adhesive layer to prevent contamination etc., until it ispractically used. Thereby, it can be prevented that foreign mattercontacts adhesive layer in usual handling. As a separator, withouttaking the above-mentioned thickness conditions into consideration, forexample, suitable conventional sheet materials that is coated, ifnecessary, with release agents, such as silicone type, long chain alkyltype, fluorine type release agents, and molybdenum sulfide may be used.As a suitable sheet material, plastics films, rubber sheets, papers,cloths, no woven fabrics, nets, foamed sheets and metallic foils orlaminated sheets thereof may be used.

In addition, ultraviolet absorbing property may be given to theabove-mentioned each layer, such as a pressure sensitive adhesive layer,using a method of adding ultraviolet absorbents, such as salicylic acidester type compounds, benzophenol type compounds, benzotriazol typecompounds, cyano acrylate type compounds, and nickel complex salt typecompounds.

A linearly polarizer of the present invention can be preferably used inmanufacture of various kinds of apparatuses such as a liquid crystaldisplay and others. Assembling of a liquid crystal display may becarried out according to conventional methods. That is, a liquid crystaldisplay is generally manufactured by suitably assembling several partssuch as a liquid crystal cell, optical elements and, if necessity,lighting system, and by incorporating driving circuit. In the presentinvention, except that a linearly polarizer by the present invention isused; there is especially no limitation to use any conventional methods.Also any liquid crystal cell of arbitrary type, such as TN type, and STNtype, π type may be used.

Suitable liquid crystal displays, such as liquid crystal display withwhich the above-mentioned linearly polarizer has been located at oneside or both sides of the liquid crystal cell, and with which abacklight or a reflector is used for a lighting system may bemanufactured. In this case, the linearly polarizer by the presentinvention may be installed in one side or both sides of the liquidcrystal cell. When installing the linearly polarizers in both sides,they may be of the same type or of different type. Furthermore, inassembling a liquid crystal display, suitable parts, such as diffusionplate, anti-glare layer, antireflection film, protective plate, prismarray, lens array sheet, optical diffusion plate, and backlight, may beinstalled in suitable position in one layer or two or more layers.

EXAMPLES

Hereinafter, the present invention is described in more detail byreference to the Examples and Comparative Examples, but the presentinvention is not limited to these examples.

In the Examples, the following compound was used as the polymerizablemesogen compound (a).

The molar absorption coefficient was 220 dm³·mol⁻¹·cm⁻¹ at 365 nm. And,as the polymerizable chiral agent (b), LC756 manufactured by BASF Ltd.was used.

Example 1

A methyl ethyl ketone solution (solids content 20% by weight) of amixture consisting of 96 parts by weight of the polymerizable mesogencompound (a) shown above, 4 parts by weight of the polymerizable chiralagent (b) described above, and 5 parts by weight of Irgacure 184(manufactured by Ciba Specialty Chemicals Co.) as thephotopolymerization initiator (c) was prepared. The solution was castonto a stretched polyethylene terephthalate substrate and dried at 80°C. for 2 minutes to remove the solvent; subsequently anotherpolyethylene terephthalate substrate was laminated thereon. Subsequentlyunder heating at 120° C., the sample was then irradiated with 3 mW/cm²ultraviolet light for 5 minutes to give the objective cholesteric liquidcrystal film.

The polyethylene terephthalate substrate in the side opposite to theside of ultraviolet light irradiation was removed. A reflection spectrumof this cholesteric liquid crystal film (circularly polarizing plate) isshown in FIG. 2. The circularly polarizing plate had an excellentproperty of separating circularly polarized light (reflection band) inthe range of 430 to 860 nm. The total thickness of the cholestericliquid crystal layer (film) was about 9 μm. The pitch length of theresulting cholesteric liquid crystal layer was 0.52 μm in the vicinityof the surface irradiated with ultraviolet light (layer lower by 1 μmfrom the surface irradiated with ultraviolet light), and 0.26 μm in thevicinity of the opposite surface (layer lower by 1 μm from the oppositesurface).

On a wide viewing angle λ/4 plate (Nz coefficient =−1.2) obtained bybiaxially stretching a polycarbonate resin film (thickness 80 μm), theresulting circularly polarizing plate was laminated via an acrylicpressure sensitive adhesive (thickness 25 μm) in such a direction as tonarrow the pitch length continuously. Further, SEG1425DU (absorptiontype polarizing plate manufactured by Nitto Denko Corporation) waslaminated thereon via a pressure sensitive adhesive, to give a broadband polarizing plate.

Example 2

A methyl ethyl ketone solution (solids content 20% by weight) of amixture consisting of 96 parts by weight of the polymerizable mesogencompound (a) shown above, 4 parts by weight of the polymerizable chiralagent (b) described above, and 0.5 parts by weight of Irgacure 907(manufactured by Ciba Specialty Chemicals Co.) as thephotopolymerization initiator (c) was prepared. The solution was castonto a stretched polyethylene terephthalate substrate and dried at 80°C. for 2 minutes to remove the solvent; subsequently anotherpolyethylene terephthalate substrate was laminated thereon. Subsequentlyunder heating at 120° C., the sample was then irradiated with 3 mW/cm²ultraviolet light for 5 minutes to give the objective cholesteric liquidcrystal film.

The polyethylene terephthalate substrate in the side opposite to theside of ultraviolet light irradiation was removed. A reflection spectrumof this cholesteric liquid crystal film (circularly polarizing plate) isshown in FIG. 3. The circularly polarizing plate had an excellentproperty of separating circularly polarized light (reflection band) inthe range of 470 to 920 nm. The total thickness of the cholestericliquid crystal layer (film) was about 10 μm. The pitch length of theresulting cholesteric liquid crystal layer was 0.56 μm in the vicinityof the surface irradiated with ultraviolet light (layer lower by 1 μmfrom the surface irradiated with ultraviolet light), and 0.28 μm in thevicinity of the opposite surface (layer lower by 1 μm from the oppositesurface).

On a wide viewing angle λ/4 plate (Nz coefficient =−1.2) obtained bybiaxially stretching a polycarbonate resin film (thickness 80 μm), theresulting circularly polarizing plate was laminated via an acrylicpressure sensitive adhesive (thickness 25 μm) in such a direction as tonarrow the pitch length continuously. Further, SEG1425DU (absorptiontype polarizing plate manufactured by Nitto Denko Corporation) waslaminated thereon via a pressure sensitive adhesive, to give a broadband polarizing plate.

Example 3

A methyl ethyl ketone solution (solids content 20% by weight) of amixture consisting of 96 parts by weight of the polymerizable mesogencompound (a) shown above, 4 parts by weight of the polymerizable chiralagent (b) described above, and 0.05 parts by weight of Irgacure 369(manufactured by Ciba Specialty Chemicals Co.) as thephotopolymerization initiator (c) was prepared. The solution was castonto a stretched polyethylene terephthalate substrate and dried at 80°C. for 2 minutes to remove the solvent, subsequently anotherpolyethylene terephthalate substrate was laminated thereon. Subsequentlyunder heating at 120° C., the sample was then irradiated with 11 mW/cm²ultraviolet light for 5 minutes to give the objective cholesteric liquidcrystal film.

The polyethylene terephthalate substrate in the side opposite to theside of ultraviolet light irradiation was removed. A reflection spectrumof this cholesteric liquid crystal film (circularly polarizing plate) isshown in FIG. 4. The circularly polarizing plate had an excellentproperty of separating circularly polarized light (reflection band) inthe range of 490 to 890 nm. The total thickness of the cholestericliquid crystal layer (film) was about 9 μm. The pitch length of theresulting cholesteric liquid crystal layer was 0.54 μm in the vicinityof the surface irradiated with ultraviolet light (layer lower by 1 μmfrom the surface irradiated with ultraviolet light), and 0.30 μm in thevicinity of the opposite surface (layer lower by 1 μm from the oppositesurface).

The resulting circularly polarizing plate was laminated on a λ/4 plate(NRF film, Δn=140 nm and Nz coefficient =−1.2, manufactured by NittoDenko Corporation) in such a direction as to narrow the pitch lengthcontinuously, and a λ/2 plate (NRZ film, Δn=270 nm and Nzcoefficient=0.5, viewing angle property compensation type, manufacturedby Nitto Denko Corporation) was arranged thereon at 117.5:° relative toan axial angle of the λ/4 plate. An acrylic pressure sensitive adhesive(thickness 25 μm) was used in each lamination. The transmissionpolarization axis in this case was 100 relative to the axis of the λ/4plate, and along this direction, SEG1425DU (absorption-type polarizermanufactured by Nitto Denko Corporation) was laminated thereon in a samemanner to give a broad band polarizing plate.

Comparative Example 1

A methyl ethyl ketone solution (solids content 20% by weight) of amixture consisting of 96 parts by weight of the polymerizable mesogencompound (a) shown above, 4 parts by weight of the polymerizable chiralagent (b) described above, and 5 parts by weight of Irgacure 184(manufactured by Ciba Specialty Chemicals Co.) as thephotopolymerization initiator (c) was prepared. The solution was castonto a stretched polyethylene terephthalate substrate and dried at 80°C. for 2 minutes to remove the solvent; subsequently anotherpolyethylene terephthalate substrate was laminated thereon. Subsequentlyunder heating at 80° C., the sample was then irradiated with 50 mW/cm²ultraviolet light for 5 minutes to give the objective cholesteric liquidcrystal film.

The polyethylene terephthalate substrate in the side opposite to theside of ultraviolet light irradiation was removed. A reflection spectrumof this cholesteric liquid crystal film (circularly polarizing plate) isshown in FIG. 5. The circularly polarizing plate had an excellentproperty of separating circularly polarized light (reflection band) inthe range of 690 to 840 nm. The total thickness of the cholestericliquid crystal layer (film) was about 9 μm. The pitch length of theresulting cholesteric liquid crystal layer was 0.50 μm in the vicinityof the surface irradiated with ultraviolet light (layer lower by 1 μmfrom the surface irradiated with ultraviolet light), and 0.50 μm in thevicinity of the opposite surface (layer lower by 1 μm from the oppositesurface).

On a wide viewing angle λ/4 plate (Nz coefficient =−1.2) obtained bybiaxially stretching a polycarbonate resin film (thickness 80 μm), theresulting circularly polarizing plate was laminated via an acrylicpressure sensitive adhesive (thickness 25 μm) in such a direction as tonarrow the pitch length continuously. Further, SEG1425DU (absorptiontype polarizing plate manufactured by Nitto Denko Corporation) waslaminated thereon via a pressure sensitive adhesive, to give a broadband polarizing plate.

Comparative Example 2

A methyl ethyl ketone solution (solids content 20% by weight) of amixture consisting of 96 parts by weight of the polymerizable mesogencompound (a) shown above, 4 parts by weight of the polymerizable chiralagent (b) described above, and 5 parts by weight of Irgacure 184(manufactured by Ciba Specialty Chemicals Co.) as thephotopolymerization initiator (c) was prepared. The solution was castonto a stretched polyethylene terephthalate substrate and dried at 80°C. for 2 minutes to remove the solvent; subsequently anotherpolyethylene terephthalate substrate was laminated thereon. Subsequentlyunder heating at 40° C., the sample was then irradiated with 3 mW/cm²ultraviolet light for 5 minutes to give the objective cholesteric liquidcrystal film.

The polyethylene terephthalate substrate in the side opposite to theside of ultraviolet light irradiation was removed. A reflection spectrumof this cholesteric liquid crystal film (circularly polarizing plate) isshown in FIG. 6. The circularly polarizing plate had an excellentproperty of separating circularly polarized light (reflection band) inthe range of 720 to 860 nm. The total thickness of the cholestericliquid crystal layer (film) was about 9 μm. The pitch length of theresulting cholesteric liquid crystal layer was 0.52 μm in the vicinityof the surface irradiated with ultraviolet light (layer lower by 1 μmfrom the surface irradiated with ultraviolet light), and 0.52 μm in thevicinity of the opposite surface (layer lower by 1 μm from the oppositesurface).

On a wide viewing angle λ/4 plate (Nz coefficient =−1.2) obtained bybiaxially stretching a polycarbonate resin film (thickness 80 μm), theresulting circularly polarizing plate was laminated via an acrylicpressure sensitive adhesive (thickness 25 μm) in such a direction as tonarrow the pitch length continuously. Further, SEG1425DU (absorptiontype polarizing plate manufactured by Nitto Denko Corporation) waslaminated thereon via a pressure sensitive adhesive, to give a broadband polarizing plate.

Comparative Example 3

A methyl ethyl ketone solution (solids content 20% by weight) of amixture consisting of 96 parts by weight of the polymerizable mesogencompound (a) shown above, 4 parts by weight of the polymerizable chiralagent (b) described above, 5 parts by weight of Irgacure 184(manufactured by Ciba Specialty Chemicals Co.) as thephotopolymerization initiator (c) and 1 part of Tinuvin 400(manufactured by Ciba Specialty Chemicals Co., ultraviolet absorbent)was prepared. The solution was cast onto a stretched polyethyleneterephthalate substrate and dried at 80° C. for 2 minutes to remove thesolvent; subsequently another polyethylene terephthalate substrate waslaminated thereon. Subsequently under heating at 80° C., the sample wasthen irradiated with 50 mW/cm² ultraviolet light for 5 minutes to givethe objective cholesteric liquid crystal film.

The polyethylene terephthalate substrate in the side opposite to theside of ultraviolet light irradiation was removed. A reflection spectrumof this cholesteric liquid crystal film (circularly polarizing plate) isshown in FIG. 7. The circularly polarizing plate had an excellentproperty of separating circularly polarized light (reflection band) inthe range of 715 to 860 nm. The total thickness of the cholestericliquid crystal layer (film) was about 9 μm. The pitch length of theresulting cholesteric liquid crystal layer was 0.51 μm in the vicinityof the surface irradiated with ultraviolet light (layer lower by 1 μmfrom the surface irradiated with ultraviolet light), and 0.51 μm in thevicinity of the opposite surface (layer lower by 1 μm from the oppositesurface).

On a wide viewing angle λ/4 plate (Nz coefficient =−1.2) obtained bybiaxially stretching a polycarbonate resin film (thickness 80 μm), theresulting circularly polarizing plate was laminated via an acrylicpressure sensitive adhesive (thickness 25 μm) in such a direction as tonarrow the pitch length continuously. Further, SEG1425DU (absorptiontype polarizing plate manufactured by Nitto Denko Corporation) waslaminated thereon via a pressure sensitive adhesive, to give a broadband polarizing plate.

The (broad band) polarizing plates obtained in the Example andComparative Examples were evaluated as follows. The results are shown inTable 1.

(Brightness Enhancement Rate)

Brightness was measured when the (broad band) polarizing plate wasamounted on a 15-inch TFT liquid crystal display, by using BM7manufactured by TOPCON CORPORATION. The Brightness enhancement rate wasmagnification calculated from improved brightness as compared withbrightness in the absence of the broad band cholesteric liquid crystalfilm.

(Viewing Angle Property: Color Tone Change)

The viewing angle property was evaluated with the following criteria, onthe basis of Δxy determined by a viewing angle measuring meterEZ-CONTRAST manufactured by ELDIM Inc.Δxy=((x ₀ −x ₁)²+(y ₀ −y ₁)²)^(0.5)

Front chromaticity (x₀, y₀), 60°chromaticity (x₁, y₁)

Good: Color tone change Δxy at a viewing angle of 60°is 0.04 or less.

Not good: Color tone change Δxy at a viewing angle of 60°is 0.04 ormore.

(Durability)

The broad band polarizing plate or the polarizing plate was placed for500 hours in an environment of 80° C. and 60° C., 90% RH, and then thepresence or absence of powdery substance precipitated on the surface wasjudged with naked eyes. When powdery substance is present, the plate isproblematic in use for optical purposes.

TABLE 1 Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 1 Ex. 2 Ex. 3 Liquidcrystal Polymerizable mesogen compound (a) 96 96 96 96 96 96 mixturePolymerizable chiral agent (b) 4 4 4 4 4 4 Photopolymerization Irgacure184 5 0 0 5 5 5 initiator (c) Irgacure 907 0 0.5 0 0 0 0 Irgacure 369 00 0.05 0 0 0 Ultraviolet absorbent (Tinuvin 400) 0 0 0 0 0 1 Solvent(methyl ethyl ketone) 400 400 400 400 400 400 Polymerization Ultravioletlight intensity (mW/cm²) 3 3 11 50 3 50 conditions Temperature (° C.)120 120 120 80 40 80 Cholesteric Reflection band (nm) 430-860 470-920490-890 690-840 720-860 715-860 liquid crystal Reflection bandwidth (nm)430 450 400 150 140 145 film Pitch length (μm) Surface 0.52 0.56 0.540.50 0.52 0.51 irradiated with ultraviolet light Opposite surface 0.260.28 0.30 0.50 0.52 0.51 Evaluation Brightness enhancement rate 1.3 1.31.3 1.0 1.0 1.0 Viewing angle property/color tone change Good Good GoodNot good Not good Not good Durability (presence or absence of AbsentAbsent Absent Absent Absent Present precipitates)

INDUSTRIAL APPLICABILITY

The broad band cholesteric liquid crystal film of the present inventionis useful as a circularly polarizing plate (reflective polarizer). Thecircularly polarizing plate can be utilized in a linearly polarizer, aluminaire, a liquid crystal display and the like.

1. A manufacturing method for a broad band cholesteric liquid crystalfilm comprising: polymerizing a liquid crystal mixture that is free ofan ultraviolet absorbent, containing a polymerizable mesogen compound(a), a polymerizable chiral agent (b) and a photopolymerizationinitiator (c) between two substrates, by irradiation of one side withultraviolet light having an intensity of 1 to 20mW/cm², wherein the twosubstrates are made of the same material, the broad band cholestericliquid crystal film has a reflection bandwidth of 300 nm or more, apitch length in the cholesteric liquid crystal film changes so as tonarrow continuously from a side irradiated with ultraviolet light, andthe pitch length is changed such that the difference in pitch lengthbetween the side of ultraviolet light irradiation and the opposite sideis made at least 100 nm.
 2. The manufacturing method for a broad bandcholesteric liquid crystal film according to claim 1, wherein thepolymerizable mesogen compound (a) has one polymerizable functionalgroup and the polymerizable chiral agent (b) has two or morepolymerizable functional groups.
 3. The manufacturing method for a broadband cholesteric liquid crystal film according to claim 1, wherein themolar absorption coefficient of the polymerizable mesogen compound (a)is 50 to 500 dm³·mol⁻¹·cm⁻¹ at 365 nm.
 4. The manufacturing method for abroad band cholesteric liquid crystal film according to claim 1, whereinthe polymerizable mesogen compound (a) is a compound represented by thefollowing general formula (1):

wherein R₁, represents a hydrogen atom or a methyl group, and a is aninteger of 1 to
 5. 5. The manufacturing method for a broad bandcholesteric liquid crystal film according to claim 1, wherein thecoating thickness of the liquid crystal mixture is in the range of from5 to 20 μm.
 6. The manufacturing method for a broad band cholestericliquid crystal film according to claim 1, wherein the polymerizationtemperature upon ultraviolet light irradiation is 140° C. or less. 7.The manufacturing method for a broad band cholesteric liquid crystalfilm according to claim 1, wherein the irradiation time of ultravioletlight is 5 minutes or less.
 8. The manufacturing method for a broad bandcholesteric liquid crystal film according to claim 1, wherein a mixingamount of the polymerizable chiral agent (b) is in the range of from 3to 7 parts by weight relative to 100 parts by weight of a total amountof the polymerizable mesogen compound (a) and the polymerizable chiralagent (b).
 9. The manufacturing method for a broad band cholestericliquid crystal film according to claim 1, wherein a mixing amount of thephotopolymerization initiator (c) is in the range of from 0.01 to 10parts by weight relative to 100 parts by weight of a total amount of thepolymerizable mesogen compound (a) and the polymerizable chiral agent(b).
 10. The manufacturing method for a broad band cholesteric liquidcrystal film according to claim 1, wherein a mixing amount of thephotopolymerization initiator (c) is in the range of from 0.05 to 5parts by weight relative to 100 parts by weight of a total amount of thepolymerizable mesogen compound (a) and the polymerizable chiral agent(b).